Are you using Yahoo or AOL? Then open your Account Settings: At the top right of the Thunderbird window, click the menu button > Options > Account Settings Select your account in the left pane - Copies and Folders - When sending messages Uncheck 'Place a copy in'

That should take care of the duplicate copy.

Wrt 'Sent emails all marked as unread' problem there is currently no fix. Also see https://support.mozilla.org/en-US/questions/1271823

I'm using AOL. I'll give your suggestion a try but - won't this put -0- copies in the Sent folder? I would like to have 1 copy of all sent messages.

MilusC said

I'm using AOL. I'll give your suggestion a try but - won't this put -0- copies in the Sent folder? I would like to have 1 copy of all sent messages.

AOL, Yahoo (and gmail and outlook.com) automatically copy sent messages to Sent, so you will still have one copy after changing the setting in TB.

I did as you suggested and it did not fix the problem. I still get 2 copies of some, but not all, sent messages put into the Sent folder. Near as I can tell, in some but not all cases, -0- copies of sent message are put into the Sent folder.

Is the account in TB POP or IMAP? Do you access the account on other devices?

You asked: "Is the account in TB POP or IMAP?"

I'm not sure but I think IMAP. The "Server Type" is "IMAP Mail Server" and the "Server Name" is "imap.aol.com".

You asked: "Do you access the account on other devices?"

When you say "the account" do you mean Thunderbird or do you mean AOL (my primary E-mail provider)? I do not access TB on any device other than my home desktop. I will sometimes access AOL on my iPhone or iPad.

Thanks for the help.

I meant if the AOL account is accessed on other devices, including phones, even though TB only runs on desktops.

There is definitely something strange with AOL sent messages. Until recently, AOL didn't automatically copy sent messages to Sent, but now it does, and the sent messages are marked as unread. Also, if attachments are deleted from sent messages, the entire message is deleted, another thing that didn't happen before.